Measuring cell for determining a paramagnetic gas

ABSTRACT

A measuring cell for use in apparatus for the measurement of the magnetic characteristics of a gas comprises means ( 5 ) defining a chamber ( 3 ) in which a test element ( 11 ) is suspended such as to be able to rotate about an axis, amd means defining inlet ( 1 ) and outlet port ( 4 ) means by which gas may flow through said chamber ( 3 ). The inlet port means is configured to cause the gas to flow in a substantially laminar flow regime. The chamber comprises a first portion ( 2 ) configured to cause the laminar gas flow to break up into a turbulent flow regime, and a second portion arranged to contain said test element. The gas flow enters from the first portion and enters the chamber in a flow pattern symmetrical in relation to the suspension position of the test element.

FIELD OF THE INVENTION

[0001] The present invention relates to apparatus for the measurement ofgases. In particular it relates to the control of a gaseous sample, e.goxygen, for introduction into a measuring system when rapid changes inthe make-up of the sample are occurring.

DESCRIPTION OF THE PRIOR ART

[0002] Within gas sensing one of the most difficult applications is theintroduction of the gas sample into apparatus used to measure thesusceptibility of a gas. Apparatus for controlling the sample introducedto such a gas sensor come in a variety of forms. The measurement of theproportion of oxygen in a sample using susceptibility has been knownsince the middle of the 19^(th) century when Faraday showed that allmaterials interacted with a magnetic field. Gases were found in generalto be repelled by a magnetic field and are described as beingdiamagnetic, whilst oxygen and some other gases were found to beattracted to a magnetic field and called paramagnetic.

[0003] Two principal methods were originally developed, Gouy (Gouy L G,Compt. Rend. Vol. 109 (1889) 935) employed a uniform magnetic fieldwhilst Selwood (Selwood P W Magnetometry 2nd Edition, 193. InterscienceN/Y London 1956) used the non-uniform field as originally described byFaraday. The bulky and delicate nature of these instruments led to thedevelopment of further apparatus amongst which the most successful werethose based on the original Faraday gas susceptibility balance. In thesedesigns a test body of well defined shape is suspended inside a gascell. Several forms of test body have been investigated including thecommonly used dumbbell by Haven (Haven G C Physical Review Vol. 4 (1932)337) with modifications using a flattened structure eg. by Gast in U.S.Pat. No. 3,815,018.

[0004] Although these methods can provide a highly accurate signal underideal gas conditions (i.e. with no gas movement) the existence of gasflow introduces errors due the extra forces created by the passage ofthe sample. For many applications the relatively slow response timesdesired, for example three to five seconds, allows a diffusion basedsample system which minimises gas flows to be employed. However, someapplications, notably pulmonary testing and breath by breath anaestheticmonitoring, have required response times in the sub second range. It hastherefore been necessary for methods of sample introduction to bedeveloped that introduce the sample into the measurement chamber suchthat the delays encountered by a purely diffusive nixing are not met. Anexample of this is given in EP 0379553, which describes a combination ofsmall cell volume and sample flow regime than can produce responsestimes of approximately 0.5 s at a flow rate of 50 ml/min.

[0005] GB-A-2196127 discloses an alternative paramagnetic sensor designwhich the gas path is arranged to have smooth transistions of constantcross section, or gently tapering or widening transitions, betweenvarious regions in order to produce a smooth unobstructed gas flow. Thisin intended to ahieve an undisrupted, non-turbulent gas flow.

[0006] The devices in the prior art do not meet all of the requirementsnow desired and in particular have limitations in the sample cell sizethat may be employed or flow errors that have to be tolerated in orderto achieve the necessary speed of response.

SUMMARY OF THE INVENTION

[0007] The present invention provides apparatus for the measurement ofthe magnetic characteristics of a gas comprising means defining achamber, a test element, means for suspending said test element in saidchamber such that said test element may rotate about an axis and meansdefining inlet and outlet port means by which gas may flow through saidchamber, wherein said inlet port means is configured to receive aninflow of gas, and to cause said gas to flow in a substantially laminarflow regime and said chamber comprises a first portion which said gasflow enters from said inlet port and which is configured to cause saidlaminar gas flow to break up into a turbulent flow regime, and a secondportion containing said test element which said gas flow enters fromsaid first portion and which is configured such that the gas flowprincipally sweeps past said element to reach said outlet port means.

[0008] The present invention therefore functions to control the flow ofgas so as to reduce components of the flow which would move themeasuring device so as to cause errors in the measurement. Thisarrangement minimises the errors, but provides a quick response time asit relies on flow to introduce the sample of the measuring chamberrather than diffusion.

[0009] The invention also provides apparatus for the measurement ofcharacteristics of a gas comprising means defining (i) a pair of inletmeans arranged to receive a flow of said gas and to cause laminar flowof said gas therein, (ii) a respective pair of expansion volumes eachbeing arranged in relation to a respective one of said inlet means toreceive said gas flow therefrom and to cause said gas flow to expand indirections normal to the direction of said laminar flow to become aturbulent flow, (iii) a measurement volume arranged to receive said flowfrom said pair of expansion volumes, and (iv) outlet means arranged toexhaust gas from said measurement volume, said apparatus furthercomprising means arranged to suspend a test element in said measurementvolume substantially symetrically in relation to said expansion volumes.

[0010] This invention therefore, in contrast to the prior art whichgenerally avoided causing turbulent flows on the basis that such flowswould introduce errors, causes and utilises turbulent flow in the gaspath, while still reducing the errors introduced into the system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] This invention will be better understood from the followingexemplary description of a preferred embodiment given with reference tothe accompanying drawings in which:

[0012]FIG. 1 is a schematic front view of the preferred device; and

[0013]FIG. 2 is a schematic side sectional view of the device of FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

[0014] The apparatus described herein allows for the measurement of agas sample in a sensor which is perturbed by rapid changes in both flowrate and gas composition, e.g. the measurement of gas susceptibilityusing a test body suspended within a non-uniform magnetic field, whereit is desired to minimise the effects of the gas flow. The principles ofthe measurement techniques are well known in the prior art above andwill not be discussed here. The important features of the new apparatuslie in how the sample is introduced into the measurement cell thatcontains the test body, and in the shaping of the flow paths to achievethis.

[0015]FIG. 1 is a schematic front view illustrating the preferredembodiment. In particular FIG. 1 shows a body 5 having within it variousflow pathways and a measuring chamber as will be discussed in detailbelow. In this embodiment, there is also a face plate, which is notshown in FIG. 1, but which closes the front of the measuring chamber.This is merely one example of the how the testing apparatus may bemanufactured.

[0016]FIG. 1 then shows a measuring chamber 3 defined within body 5having located therein a dumbell type test element 11 suspended bysuspension elements 12. Dumbell 11 is suspended such that it may rotateabout an axis defined by suspension element 12 which also function topermit an electric current to be passed to the test element 11. Body 5further includes, or has associated with it, magnets, for instance asshown schematically by magnetic portions 20, arranged to generate amagnetic field within the measurement chamber.

[0017] As is well known the deflection or movement of the test element11 about axis 12 within the magnetic field when current is applied isdependent on the magnetic susceptibility of the gas present in thechamber 3. Therefore, measurement of such movement gives a measure ofthat susceptibility. The particular methods of measurement are notdescribed here as they do not form the subject matter of the presentinvention, but suitable methods may be found in the publicationsmentioned above and other patents assigned to Servomex.

[0018] It is sufficient to note however that, for accurate measurement,it should be ensured, as far as possible, that any movement of the testelement 11 is a result of changes in the susceptibility of the gaspresent in the chamber, and/or the measurement method being used. Otherfactors causing movement of the test element may introduce errors intothe measurements produced.

[0019] It is however, also desired that the sensor should be quicklysensitive to changes in a gas flow, and this requires efficient flushingof gas through the chamber. This raises the considerable risk of causingerrors in the measurement mentioned above by the simple mechanicalaction of the gas flow moving the test element. i.e. blowing it out ofposition.

[0020] As can be seen in FIG. 1, the device further comprises two flowinlet ports 1 by which gas is introduced into the measuring chamber. Thechamber within body 5 is shaped so as to control the amount of gas flowwhich impacts the test element such as to cause errors in themeasurement mentioned above. There is also defined within body 5 anoutlet or exhaust port which is not shown in FIG. 1.

[0021]FIG. 2 is a schematic side sectional view of the device shown inFIG. 1. FIG. 2 again shows the measurement chamber 3 having test element11 suspended therein to rotate about the axis defined by suspensionelements 12. Also as shown in FIG. 2, body 5 is provided with face plate7 which completes the formation of the various flow paths to bediscussed in detail below. Finally, the exhaust port 4 is shown in FIG.2, but it should be noted that the features shown in FIGS. 1 and 2 arenot drawn to scale.

[0022] Referring then to FIG. 2, the gas sample enters via ports 1(arrows A). Inlet ports 1 are of a narrow diameter, for instance 1 mm toreduce any “smearing” of concentration changes in the gas flow., i.e. toensure that such changes are efficiently passed into the device. At thefront of the device, the gas flow is forced to undergo a ninety degreeturn (arrows B) to enter a narrow channel 2 defined between body 5 andface plate 7. This turn breaks up the gas flow and causes the flow inchannel 2 to have a wide distribution of momentum with no overall directflow. The flow is at this stage made up of many very small vortices, andthis can be considered analagous to creating a spray of fine waterdroplets at the outlet of a hosepipe by substantially obstructing theoutlet.

[0023] The gas then enters the measuring chamber 3, where the largeincrease in cross section causes the flow per unit area to fallsignificantly. The test body 11 is mounted as shown such that it liesparallel to and in symmetrical relation to the narrow channel, 2. Thegas then flows past the test element 11 (arrows C). The break up of theflow as mentioned above into many fine vortices is one factor whichreduces the force the flow applies to the test element, because theforces of the vortices average out to be zero or near zero.Additionally, the differential force in a direction which would causethe test element to rotate created by the flow across the test body isnegligible and therefore the measurement errors introduced by the flowor changes therein are further minimised.

[0024] Furthermore as the measurement chamber is actually being swept bythe gas flow, rather than relying on diffusion, any change in sampleconcentration occurs rapidly. After passing through the measuringchamber the gas enters the exhaust port 4 (arrow D) accelerating the gasflow. However the port 4 is preferably larger than the inlet ports 1(e.g. greater than 2 mm) to permit any chaotic flow to be rapidlyexpelled. Also, an extended gap at the back of the measuring chambercompared to the front reduces the effect of any turbulence generated asthe sample enters the smaller bore outlet from acting on the test bodyand thereby re-introducing the flow errors the invention alleviates.

[0025] In FIG. 1 it can be seen that even further beneficial effects canbe obtained within the present invention.

[0026] Here it can be seen that the narrow channel, mentioned above, hasa gradually increasing width. This further decelerates the gas becauseof the increasing volume and brings the gas into contact with a largesurface area thereby promoting the rapid removal of large scaleturbulence in its flow straightening function. A further feature that isapparent in FIG. 1 is that the gas flow from either of the inlet ports 1does not flow directly into the measuring chamber because of shieldingportion 6. This shielding is one of the principal factors in determiningthe balance achieved between flow error and speed of response.

[0027] In the configuration shown in FIG. 1 the gas flow into themeasuring chamber is as shown by arrow E. In this arrangement there isno or negligible gas momentum which impacts directly on the test element11. Rather the gas simply sweeps past the test element. This is the mostpreferred arrangement from the viewpoint of measurement accuracy, and infact, with this arrangement, the errors introduced by the gas flow aresmaller than the error margin in many measurement methods, meaning thatthe gas flow errors are effectively zero.

[0028] If the amount of shielding at corners 6 a is reduced which allowssome of the sample to pass directly to the that portion of themeasurement cell that lies nearest to it then the flow error willincrease but will provide an improved response time. This trade offoccurs because if the sample is allowed to flow significantly throughthe measurement chamber directly adjacent to an inlet port the flow willtend to flow asymmetrically around the test body. i.e. more flow passesbetween the nearer wall and the body than the further, producing animbalanced force which leads directly to the flow error. Forcing theflow to pass through the centre point of the measurement chamber allowsthe sample to expand giving a more balanced flow either side of the testbody.

[0029] Within this trade off it is possible to configure the testapparatus to be sensitive to changes of a few tenths of one percentoxygen level and with a response time less than 200 ms.

[0030] The above embodiment employs a torque balance type sensor toelucidate the function of the invention. However, the principle isapplicable to wide range of devices which are susceptible to errorsintroduced by rapid changes in sample flow or composition and as suchhas a wider application than only to torque balance sensors.

[0031] Although the above description is of one preferred embodiment, itwill be seen that there are important features of the embodiments whichassist in achieving the object of the invention. These include thebreaking up of the gas flow such that the gas flow around the testelement has no relationship with the input gas flow. This enables thegas flows in the measurement chamber to be carefully controlled. The gasflows are controlled to have a low or zero velocity vector in the planeof sensitivity of the detector element, and the overall symmetry of thedevice assists here too.

1. Apparatus for the measurement of the magnetic characteristics of agas comprising means (5) defining a chamber (3), a test element (11),means for suspending said test element in said chamber such that saidtest element may rotate about an axis (12) and means (5, 7) defininginlet and outlet port means by which gas may flow through said chamber(3), wherein said inlet port means is configured to receive an inflow ofgas, and to cause said gas to flow in a substantially laminar flowregime and said chamber comprises a first portion (2) which said gasflow enters from said inlet port and which is configured to cause saidlaminar gas flow to break up into a turbulent flow regime, and a secondportion containing said test element which said gas flow enters fromsaid first portion and which is configured such that the gas flowprincipally sweeps past said element to reach said outlet port means. 2.Apparatus according to claim 1 in which said outlet port means has across-sectional area larger than that of said inlet port means. 3.Apparatus according to claim 1 or 2 in which said inlet port meanscomprises a pair of first inlet passageways, and said first portion ofsaid chamber comprises a pair of respective expansion passageways inwhich said gas flows in said turbulent flow regime, said turbulent flowbeing substantially normal to the direction of flow in said inletpassageways.
 4. Apparatus according to claim 3 in which the meansdefining the expression passageways comprises a shielding portion (6)arranged to deflect the gas flow into the second portion of the chamberaway from a direction parallel to said axis.
 5. Apparatus according toany of claims 1-4 further comprising magnet means arranged to provide amagnetic field within said chamber.
 6. Apparatus according to any ofclaims 1-5 in which said means for suspending said test element enablesthe application of electric current to said element.
 7. Apparatusaccording to any of claims 1-6 further comprising means for measuringthe movement of the test element.
 8. A measurement cell for use inapparatus for the measurement of the magnetic characteristics of a gascomprising means (5) defining a chamber (3) in which a test element (11)may be suspended such as to be able to rotate about an axis, and meansdefining inlet and outlet port means by which gas may flow through saidchamber (3), wherein said inlet port means is configured to receive aninflow of gas and to cause said gas to flow in a substantially laminarflow regime and said chamber comprises a first portion (2) which saidgas flow enters from said inlet port and which is configured to causesaid laminar gas flow to break up into a turbulent flow regime, and asecond portion arranged to contain said test element which said gas flowenters from said first portion and which is arranged to cause the gasflow to enter said chamber in a flow pattern symmetrical in relation tothe intended suspension position of said test element.
 9. A measurementcell according to claim 8 in which said outlet port means has across-sectional area larger than that of said inlet port means.
 10. Ameasurement cell according to claim 8 or 9 in which said inlet portmeans comprises a pair of first inlet passageways, and sadi first prtionof said chamber comprises a pair of respective expansion passageways inwhich said gas flows in said turbulent flow regime, said turbulent flowbeing substantially normal to the direction of flow in said inletpassageways.
 11. A measurement cell according to claim 10 in which themeans defining the expression passageways comprises a shielding portion(6) arranged to deflect the gas flow into the second portion of thechamber away from a direction parallel to said axis.
 12. Apparatus forthe measurement of characteristics of a gas comprising means defining(i) a pair of inlet means arranged to receive a flow of said gas and tocause laminar flow of said gas therein, (ii) a respective pair ofexpansion volumes each being arranged in relation to a respective one ofsaid inlet means to receive said gas flow therefrom and to cause saidgas flow to expand in directions normal to the direction of said laminarflow to become a turbulent flow, (iii) a measurement volume arranged toreceive said flow from said pair of expansion volumes, and (iv) outletmeans arranged to exhaust gas from said measurement volume, saidapparatus further comprising means arranged to suspend a test element insaid measurement volume substantially symetrically in relation to saidexpansion volumes.